Protection circuit for an adjustable motorized hospital bed

ABSTRACT

The electrical control system for a motor operated adjustable hospital bed is energized by AC voltage from a grounded AC power supply. A separate ground connection between the bed frame or chassis and the power supply ground establishes the bed chassis at earth ground potential during normal operating conditions. The patient is protected against shock hazards in the event that he contacts an AC voltage (from the bed&#39;&#39;s control system or from any external electrical apparatus) at the same time that he touches the bed chassis, thereby causing ground leakage current to flow through his body. This protection is achieved by constantly monitoring the ground leakage current flowing through the ground connection and opening that connection anytime the leakage current exceeds a predetermined threshold level, such as around 9 microamperes. In addition, the AC power supplied to the bed is interrupted.

United States Patent 1 1 Volk, Jr.

[ PROTECTION CIRCUIT FOR AN ADJUSTABLE MOTORIZED HOSPITAL BED [75]Inventor: Joseph A. Volk, Jr., Florissant, Mo.

[73] Assignee: Borg-Wamer Corporation, Chicago,

Ill.

[22] Filed: Mar. 18, 1974 [21] Appl. No: 452,223

[52] U.S. Cl 317/18 D; 317/9 A; 317/10 [51] Int. CI. .1 HOZH 3/08; HOZH7/20 [58] Field of Search... 317/18 R, 18 D, 18 A, 18 B,

[56] References Cited UNITED STATES PATENTS 2,999,189 9/1961 Gerrard n317/18 B 3,515,942 6/1970 Gordon 317/18 D 3,525,018 8/1970 Murphy et a1.r r 317/18 D 3,579,037 5/1971 Hackman et a1 317/18 R 3,617,808 11/1971Yoder 317/18 D 3,697,808 10/1972 Lee 317/18 A Fault Indicator 20v. Powerpp y Hot Earth Ground Bed Chassis 1 1 Nov. 4, 1975 Primary Examinerl. D.Miller Assistant E.raminerPatrick R. Salce Attorney, Agent, orFirm-James E. Tracy ABSTRACT The electrical control system for a motoroperated adjustable hospital bed is energized by AC voltage from agrounded AC power supply. A separate ground connection between the bedframe or chassis and the power supply ground establishes the bed chassisat earth ground potential during nonnal operating conditions. Thepatient is protected against shock hazards in the event that he contactsan AC voltage (from the beds control system or from any externalelectrical apparatus) at the same time that he touches the bed chassis,thereby causing ground leakage current to flow through his body Thisprotection is achieved by constantly monitoring the ground leakagecurrent flowing through the ground connection and opening thatconnection anytime the leakage current exceeds a predetermined thresholdlevel, such as around 9 mi croamperes. In addition, the AC powersupplied to the bed is interrupted.

8 Claims, 1 Drawing Figure Control System For Adjustable MotorizedHospital Bed Ground PROTECTION CIRCUIT FOR AN ADJUSTABLE MOTORIZEDHOSPITAL BED BACKGROUND OF THE INVENTION An adjustable motorizedhospital bed, the positioning of which may be remotely controlled bymeans of an electrical control system operated by the patient, must becompletely safe to preclude any possibility of the patient being subjectto hazardous electrical shocks from the voltages present in the controlsystem or in the conductors supplying AC power thereto. To that end, andin accordance with conventional practice, the metal bed frame or chassisis usually grounded to the power supply ground or building earth groundso that circuit component failures or insulation breakdowns cannotestablish the bed frame at a dangerously high potential relative toearth ground. Unfortunately, even though a patient may be made immune toor protected against malfunctioning or faults that tend to cause theapplication of undesired voltages to the bed chassis, he may still besubject to lethal currents due to the surrounding electricalenvironment.

To elucidate, the patient may contact, due to a fault condition, avoltage in the beds control system, or he may come in contact with avoltage from some faulty electrical device or appliance, such as atelevision set. radio, lamp, etc. On the other hand, the patient mayreceive a voltage from medical electronic equipment at tached to thepatient for diagnostic or therapeutic purposes. Any one of theseconditions can result in lethal current passing through the patientsbody. For example, assume that one of the patients hands touches anungrounded metallic cabinet housing for a television receiver whosepower cord is frayed such that the hot line conductor is in contact withthe metallic housing. If the patient simultaneously places some otherportion of his body in contact with the grounded bed frame a circuitpath will be completed through his body, causing current to flowtherethrough which may be of a magnitude sufiicient to cause death. Ofcourse, a very dan gerous situation exists and the patient isparticularly vulnerable when he is connected to medical electronicapparatus via a catheter or an implanted electrode. It has been foundthat current flow as low as microam peres through a patients heart iscapable of causing ventricular fibrillation The present inventionconstitutes a significant improvement over the protection circuitsheretofore developed for motor operated hospital beds, since for thefirst time a patient may be totally protected from any fault or failureeither in the bed itself or in any electrical apparatus used by or onthe patient.

SUMMARY OF THE INVENTION The protection circuit of the invention is tobe incorporated in an adjustable motorized hospital bed wherein agrounded AC power supply is coupled to the beds electrical controlsystem for applying AC operating voltage thereto, and wherein a groundconnection couples the bed chassis to the power supply ground. Means areprovided for monitoring any ground leakage current flowing through theground connection, and switching means responds to the monitoring meansfor interrupting the ground connection anytime the ground leakagecurrent exceeds a predetermined threshold level.

DESCRIPTION OF THE DRAWING The features of the invention which arebelieved to be novel are set forth with particularity in the appendedclaims. The invention, together with further advantages and featuresthereof, may best be understood, however, by reference to the followingdescription in conjunction with the accompanying drawing whichschematically illustrates a protection circuit, constructed inaccordance with the invention, and the manner in which the circuit isincorporated in the electrical system of an adjustable motorizedhospital bed.

DESCRIPTION OF THE ILLUSTRATED EMBODIMENT Block 10 represents aconventional grounded AC power supply providing a single-phasealternating voltage having a magnitude of approximately volts RMS and acommutating frequency of 60 cycles per second or hertz. This AC voltage,or what is commonly called line voltage, is usually available at a walloutlet in any hospital room where an adjustable hospital bed is located.The line voltage is produced across the output terminals to which theconductors labeled Hot and Neutral are connected. The third outputterminal of power supply 10 is connected to the building ground or earthground, as is also the case with the Neutral output terminal. Inconventional manner, the voltage at the Hot terminal will alternate ingenerally sinusoidal fashion above (or positive) and below (or negative)relative to the earth ground reference plane.

Under normal conditions the line voltage is applied to the primarywinding 14 of an isolation transformer 15 through a series-connectedbidirectional semiconductor or solid state switching device 18 whichpreferably takes the form of a triac having first and second mainterminals, labeled T and T respectively, and a control or gate terminalG. A triac may be considered as two parallel PNPN structures oriented inopposite directions to provide symmetrical bidirectional electricalcharacteristics to permit current flow between the main terminals ineither direction. It operates basically as two silicon controlledrectifiers or SCRs connected in parallel, but with the anode and cathodeof one SCR connected to the cathode and anode. respectively, of theother SCR. In the absence of any applied voltages, a triac assumes itsOff condition in which a very high impedance exists between mainterminals T and T to effectively constitute an open switch. When avoltage of either polarity is impressed across the main terminals at thesame time that triggering or gate current of appropriate magnitude flowsbetween terminals G and T, in either direction, the triac turns ON topermit current flow between terminals T, and T in response to thevoltage applied thereto and in the direction determined by the voltagespolarity. When the triac conducts, a very low impedance is presentedbetween its main terminals so that it essentially functions as a closedswitch.

Isolation transformer 15, while not essential to the operation of thepresent invention, isolates the beds electrical system from the ACvoltage supplied over the neutral and hot conductors. Of course, itprovides no isolation with respect to any voltage that the patient mayencounter from some external electrical apparatus also powered by powersupply 10. Of course, the AC supply will usually be common to all theelectrical equipment in the hospital room. Inasmuch as transformer I5merely serves to isolate the beds electrical system from power supply10, it preferably has a lzl turns ratio. The terminals of secondarywinding 16 connect over respective ones of line conductors L, and L tothe electrical control system for the adjustable mo torized hospitalbed, depicted in the drawing by the single block labeled 21. Undernormal operating conditions when there are no circuit component failuresand when the patient does not come in contact with a voltage andprovides a path for ground leakage current. the line voltage of 120volts is applied. via transformer 15, to control system 21 to effectenergization thereof in order that the positioning of the hospital bedmay be under control of the patient occupying the bed.

In a typical arrangement, the control system includes a motor-drivesystem and a pushbutton-actuated control device capable of remotelycontrolling the motordrive system. By selectively depressing thepushbuttons. the patient is able to control the various adjustments.Usually. the mattress-supporting structure of the bed is articulated,being divided into four interconnected sections or panels, namely a backsection, a center or seat section, an upper knee or thigh section and alower knee or foot section. One motor-driven adjustment that may becontrolled by the patient raises or lowers the two knee sections wherethey join together, thereby controlling the position of the patientsknees. Another adjustment. under the patients control, pivots or tiltsthe back section with respect to the center section so that thepatient's back and head may be raised or lowered. In most cases, a thirdmotor-driven adjustment may be controlled by the pushbutton-actuatedcontrol device to vertically adjust the entire mattresssupporting frame.

As one example of a construction that control system 21 may take,reference is made to copending patent application Serv No. 380,310,filed July 18, 1973 in the name of Kenneth W. Padgitt. In thatapplication, optically coupled circuits are employed to electricallyisolate the patients hand control unit from the much higher currentmotor-drive circuitry.

The metal bed chassis or frame, which of course is conductive, isgrounded to the power supply ground through a ground connection thatincludes a seriesconnected resistor 23 and a series-connected solidstate switch 24, preferably in the form of a triac. As is the case withtriac l8, triac 24 is normally maintained in its ON condition so thatthe ground connection is continuous and ground leakage current may flowto earth ground. Preferably, resistor 23 has a resistance of 500 ohmsbut by shunting it with the oppositely poled diodes 24 and 25, theapparent resistance is reduced to approximately 0.l ohm at voltagesgreater than one volt peak-to-peak. In other words, when the voltageacross resistor 23 reaches a magnitude sufficient to turn diodes 24 and25 ON, the ground current flows primarily through the diodes. Capacitor27 merely serves as an RF (or radio frequency) bypass so that operationof the protection system will be limited to the low frequency (60 hertz)of the AC power supply. To explain. if, for example, RF radiation from atelevision set is picked up by the ground connection, the RF signal willbe shunted through capacitor 27 so that no voltage develops acrossresistor 23.

A DC power supply 29 is coupled to the neutral and hot outputs of mainAC power supply in order to produce a DC voltage, preferably around l0volts, for operating the protection circuit. Of course. all of theterminals in the drawing marked are tied to the positive output of DCsupply 29. A bistable device, such as a bistable multivibrator or flipflop 31, controls triacs l8 and 24. Flip flop 31 has two stableoperating conditions. designated for convenience as the set and resetconditions respectively, Once the flip flop is triggered to its resetcondition by means of a triggering pulse applied over its reset input,the flip flop will remain in that condition subsequent to thetermination of that pulse and until another triggering pulse is appliedover its set input, whereupon it will be actuated to its set condition,where it will remain until reset.

A reset circuit automatically resets the flip flop when power supply 29becomes energized. At that time, capacitor 32 charges through resistor33 toward the 10 volts positive potential of DC source 29. Thepositivegoing voltage at the junction of capacitor 32 and resistor 33 isthreshold detected in amplifier 35 to produce a suitable reset pulsewhich is applied via resistor 36 to the reset input of flip flop 31.Diode 37 provides a discharge path for capacitor 32 when power isremoved from the circuit. For reasons to be appreciated, a manual resetis also provided for flip flop 31. Manually operated reset switch 38 isa normally open pushbutton momentary contact switch. By depressingswitch 38, the positive voltage of DC supply 29 is applied to thedifferentiating circuit formed by resistor 39 and capacitor 41 which inturn produces a triggering pulse for resetting the flip flop.

The two outputs 43 and 44 of flip flop 31 produce phase opposed,rectangular wave signals each of which varies between positive andnegative amplitude levels, relative to the earth ground reference plane,as the flip flop is actuated between its set and reset conditions. Thetwo output signals are amplified by a pair of buffer amplifiers 45 and46 such that their output voltages will be positive and negative,respectively, when flip flop 31 is in its reset condition, whereas theoutputs of those buffers will be reversed when the flip flop is in itsset condition.

With flip flop 31 in its reset condition, the positive output voltage ofbuffer 45 is applied via current limiting resistor 48 to the base of NPNtransistor 49 to turn it ON and effect current flow through lamp S1 ofopto isolator 5 2. The lamp illuminates and causes photo resistor 53 ofthe opto isolator to exhibit a resistance sufficiently low to permitgate current to flow through resistor 55 and between the gate and mainterminal T of triac 18, as a result of which the triac turns ON andcompletes the coupling between AC supply 10 and primary 14 so that poweris supplied to the electrical control system 21. Simultaneously, thenegative output voltage of buffer 46 is applied via current limitingresistor 56 to the base of NPN transistor 57 to maintain that transistorin its OFF condition. This permits the gate of traic 24 to receive gatecurrent through biasing resistors 58 and 59 to maintain the triacconductive and the ground wire continuous.

Hence, under normal conditions flip flop 31 will be established in itsreset condition and this in turn results in both of the triacs being intheir ON states so that the AC power supply will be coupled to controlsystem 21 and at the same time the ground connection between the chassisand earth ground remains uninterrupted.

It will now be shown that in the event of any one of a variety ofdifferent faults or malfunctions in either the beds electrical system orin any external electrical equipment or device that the patient engages,flip flop 31 will be triggered to its set condition to disconnect the ACpower supply from control system 21 and to interrupt the groundconnection. Initially, the protection circuit responds to any componentfailure or breakdown that effects the application of an undesirablevoltage to the bed frame. To that end, a pair of seriesconnected,oppositely poled diodes 61 and 62 are connected in shunt with secondarywinding 16 of the isolation transformer, the junction of the diodesbeing cou pled to the ground connection via a resistor 63 and acapacitor 65.

During normal operation when no fault exists, no current flows from theisolated secondary circuit and through resistor 63 and capacitor 65 dueto the presence of series-opposed diodes 61 and 62. Specifically, duringthe half cycles when line conductor L, is positive relative to conductorL diode 62 blocks any current flow, whereas during the alternate halfcycles when line conductor L is positive with respect to conductor Ldiode 61 prevents the flow of current. When a fault occurs in thecontrol system such that one of its components fails and in so doingestablishes a lowimpedance coupling (either capacitive. resistive,induc' tive or direct short) between the bed frame and either one ofline conductors L, and L pulsating DC fault current flows between thejunction of diodes 61 and 62 and the ground connection. Consider, forexample, that conductor L becomes coupled to the bed chassis. Diode 61thus becomes short circuited with the result that current flows fromline L; and through diode 62, resistor 63 and capacitor 65 during thehalf cycles when line L is positive relative to line L,. Of course, themagnitude of this fault current is dependent on the impedance in theundesirable coupling between line L, and the bed frame.

Resistor 67 forms a filter with capacitor 65 to develop at circuitjunction 68 a positive DC voltage, of an amplitude proportional to thefault current, for application to the set input of flip flop 31.Assuming that the fault current exceeds a given threshold level, such as9 microamperes, this positive DC voltage will be sufficient to triggerflip flop 31 from its reset to its set condition. Diode 71 couplesjunction 68 to the positive output of DC power supply 29 to insure thatthe voltage applied to the set input never exceeds volts. When the flipflop sets, the potentials at outputs 43 and 44 re verse their polaritiessuch that buffer 45 now applies a negative voltage to the base oftransistor 49 and buffer 46 impresses a positive voltage on the base oftransistor 57.

Transistor 49 thus turns OFF to extinguish lamp 51, whereupon photoresistor 53 introduces a resistance in the gate circuit for triac l8sufficiently high to render the triac non-conductive. Primary winding 14thus becomes disconnected from power supply 10. At the same time. thepositive voltage applied to transistor 57 turns that transitor ON.shorting out the gate current for traic 24 to cause it to become cutoff,as a consequence of which the ground connection interrupts. When triac[8 is switched OFF. the full volts is applied to the fault indicatorcircuit comprising the lamp 72 (which may be a neon bulb) and currentlimiting resistor 73. The lamp illuminates at this time to provide avisual signal that a malfunction or failure exists. When the troublesomecomponent is located and repaired or replaced, reset switch 38 may bedepressed to reset the flip flop and reestablish the application of linevoltage to primary winding 14 and at the same time to recon- 6 nect theground connection between the bed frame and the power system ground.

Assume now that the patient comes in contact with the hot output ofpower supply 10 due to malfunction or fault in the beds electricalcircuits or in some external equipment. As noted, power supply 10 willusually be common to all of the electrical apparatus in the hospitalroom where the bed and patient are located. If some other part of thepatients body then touches the bed chassis, a circuit path will becompleted through the patient from the hot line to earth ground.However, in accordance with a salient feature of the invention thepatient will be protected from the ground leakage current and he willsuffer no injury whatsoever, even though the circuit path may includethe patients heart. Total patient protection is achieved by constantlymonitoring any ground leakage current flowing through the patients bodyand the ground connection (which will be in series) and opening theground connection as soon as any such current exceeds a predeterminedthreshold level, preferably 9 microamperes.

To explain, by providing resistor 23, shunted by diodes 24 and 25, inseries with the ground connection, ground leakage current may bemonitored by-sensing the voltage across the resistor. When such currentflows, the AC voltage across resistor 23 is applied via a couplingcapacitor to amplifier 76 which is bridged by a resistor 77 to obtainnegative feed back to establish the amplifiers gain characteristic. Theamplified AC output signal is applied through coupling capacitor 78 topotentiometer 79 where the signal is provided at adjustable tap 81 at anamplitude depending on the positioning of the tap. It is then suppliedvia coupling capacitor 82 to another amplifier 83 for furtheramplifcation, resistor 84 introducing negative feedback for establishingthe amplifiers gain characteristic. The amplified ground leakage signal,developed by amplifier 83, is applied through capacitor 85 to diodes 86and 87 and resistor 88. The diodes provide a peak-to-peak detector toclamp the negative peaks of the amplified signal to ground and torectify the resulting signal, as a consequence of which a positivevoltage is developed to which capacitor 65 charges through resistor 88.A positive voltage is thus produced at circuit junction 68 having anamplitude proportional to the amount of ground leakage current flowingthrough the ground connection. By appropriately adjusting potentiometer79, the sensitivity of the protection circuit may be varied. Preferably,it is adjusted so that a ground leakage current as low as 9 microamperesproduces a positive voltage at junction 68 sufi'icient to actuate flipflop 31 to its set condition which, of course. results in turning OFFtriacs l8 and 24 to decouple the AC power supply from control system 21and to open the ground lead. Setting the threshold level at around 9microamperes insures that the circuit path through the patient will bebroken before any injury is sustained. The protection circuit may easilybe constructed so that it trips and opens the ground connection within0.0]6 second.

The presence of back to back diodes 24 and 25 allows resistor 23 to bebypassed for heavy currents in the event the protection circuitmalfunctions. Assume, for example, that triac 24 fails and becomespermanently shorted between its main terminals. The low impedence pathprovided by the diodes establishes a good ground connection in order toprotect the external equipment frtliii any damage to their components.

Of course, while triacs are preferred for interrupting the hot line andthe ground lead, a wide variety of switching devices could be usedinstead. For example, relays could be employed. n the other hand, alight emitting diode photo coupler may be employed. As still anotherexample, each switch may comprise two NPN transistors whoseemitter-collector paths are in series with each other and with theground connection, the emitter of one transitor being directly connectedto the emitter of the other. Turning both of the transistors ONcompletes the ground connection. With this arrangement, the offset biasvoltage of one transistor is balanced by the bias voltage of the otherso that there is no voltage drop between the two collectors.

The invention provides, therefore, a novel protection circuit fordetecting anytime that ground leakage current, above a predeterminedthreshold level, flows through a patient's body, whereupon thecurrent-carrying path is immediately opened to protect the patient fromshock hazards and serious injury. In addition, the protection circuitdetects when ever line voltage is applied to the bed frame, in whichevent the AC power supply automatically disconnects from the bed.

While a particular embodiment of the invention has been shown anddescribed, modifications may be made, and it is intended in the appendedclaims to cover all such modifications as may fall within the truespirit and scope of the invention.

1 claim:

I. A protection circuit for an adjustable motorized hospital bed whereina grounded AC power supply is coupled to the bed 5 electrical controlsystem for applying AC operating voltage thereto, and wherein a groundconnection couples the bed chassis to the power supply ground,comprising:

means, including a series-connected resistor in the ground connection,for monitoring any ground leakage current flowing through said groundconnection;

switching means responsive to said monitoring means for interrupting theground connection anytime the ground leakage current exceeds apredetermined threshold level;

and a pair of oppositely poled diodes, each connected in shunt with saidresistor, for limiting the voltage developed across said resistor.

2. A protection circuit according to claim 1 in which said switchingmeans also decouples the beds electrical control system from the ACpower supply when the ground leakage current exceeds said predeterminedthreshold level.

3. A protection circuit according to claim 2 wherein the AC operatingvoltage is applied to the bed s electrical control system over a pair ofline conductors, and including means for actuating said switching meansto decouple the control system from the AC power supply anytime eitherone of the line conductors becomes grounded to the bed chassis.

4. A protection circuit according to claim 2 wherein the AC power supplyis coupled to the bed s electrical control system via an isolationtransformer, the primary winding of which is disconnected from the ACpower supply by said switching means when the ground leakage currentexceeds said predetermined threshold level.

5. A protection circuit according to claim 4 and including a pair ofseries-connected, oppositely poled diodes in shunt with the secondarywinding of said transformer, the junction of said diodes being coupledto said ground connection, wherein said monitoring means monitors anyfault current flowing between said junction and said ground connection,and in which said switching means responds to said monitoring means todecouple the bed's electrical control system from the AC power supplyanytime the fault current exceeds a given threshold level.

6. A protection circuit according to claim 4 wherein the primary windingof said isolation transformer is coupled to the AC power supply via hotand neutral leads, and wherein a pair of solid state switches in serieswith the hot lead and the ground connection, respectively, are renderednon-conductive when the ground leakage current exceeds saidpredetermined threshold level.

7. A protection circuit according to claim 1 in which said switchingmeans is actuated by said monitoring means to interrupt the groundconnection when the ground leakage current exceeds approximately 9microamperes.

8. A protection circuit according to claim 1 in which said switchingmeans includes a solid state switch in series with the groundconnection, a bistable device for controlling said solid state switchand having set and reset operating conditions, and means forestablishing said bistable device normally in its reset operatingcondition to render said solid state switch conductive, therebymaintaining the ground connection continuous; and in which saidmonitoring means actuates said bistable device to its set condition,when the ground leakage current exceeds said predetermined thresholdlevel, to render said solid state switch non-conductive and to interruptthe ground connection.

1. A protection circuit for an adjustable motorized hospital bed whereina grounded AC power supply is coupled to the bed''s electrical controlsystem for applying AC operating voltage thereto, and wherein a groundconnection couples the bed chassis to the power supply ground,comprising: means, including a series-connected resistor in the groundconnection, for monitoring any ground leakage current flowing throughsaid ground connection; switching means responsive to said monitoringmeans for interrupting the ground connection anytime the ground leakagecurrent exceeds a predetermined threshold level; and a pair ofoppositely poled diodes, each connected in shunt with said resistor, forlimiting the voltage developed across said resistor.
 2. A protectioncircuit according to claim 1 in which said switching means alsodecouples the bed''s electrical control system from the AC power supplywhen the ground leakage current exceeds said predetermined thresholdlevel.
 3. A protection circuit according to claim 2 wherein the ACoperating voltage is applied to the bed''s electrical control systemover a pair of line conductors, and including means for actuating saidswitching means to decouple the control system from the AC power supplyanytime either one of the line conductors becomes grounded to the bedchassis.
 4. A protection circuit according to claim 2 wherein the ACpower supply is coupled to the bed''s electrical control system via anisolation transformer, the primary winding of which is disconnected fromthe AC power supply by said switching means when the ground leakagecurrent exceeds said predetermined threshold level.
 5. A protectioncircuit according to claim 4 and including a pair of series-connected,oppositely poled diodes in shunt with the secondary winding of saidtransformer, the junction of said diodes being coupled to said groundconnection, wherein said monitoring means monitors any fault currentflowing between said junction and said ground connection, and in whichsaid switching means responds to said monitoring means to decouple thebed''s electrical control system from the AC power supply anytime thefault current exceeds a given threshold level.
 6. A protection circuitaccording to claim 4 wherein the primary winding of said isolationtransformer is coupled to the AC power supply via hot and neutral leads,and wherein a pair of solid state switches in series with the hot leadand the ground connection, respectively, are rendered non-conductivewhen the ground leakage current exceeds said predetermined thresholdlevel.
 7. A protection circuit according to claim 1 in which saidswitching means is actuated by said monitoring means to interrupt theground connection when the ground leakage current exceeds approximately9 microamperes.
 8. A protection circuit according to claim 1 in whichsaid switching means includes a solid state switch in series with theground connection, a bistable device for controlling said solid stateswitch and having set and reset operating conditions, and means forestablishing said bistable device normally in its reset operatingcondition to render said solid state switch conductive, therebymaintaining the ground connection continuous; and in which saidmonitoring means actuates said bistable device to its set condition,when the ground leakage current exceeds said predetermined thresholdlevel, to render said solid state switch non-conductive and to interrupTthe ground connection.